John Bokma's Hacking & Hiking

Hand coding an RSS 2.0 feed in Perl

October 9, 2019

One requirement I have for tumblelog is that everything the Perl version generates is identical to everything the Python version generates. This means that sometimes I have to hand code a function that is available in a library for, say Python, but works differently in a Perl library.

When I was working on adding an RSS feed to the Perl version I decided to use XML::RSS at first. But I didn't like the generated output, also because I had little to no control over it. Next I have XML::Writer a spin, but couldn't match the output of Python, using lxml.etree. So I decided to hand code both the Perl and the Python version as it's a simple feed, and XML::Writer just added a few wrapper functions.

my @MON_LIST = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
my @DAY_LIST = qw( Sun Mon Tue Wed Thu Fri Sat );

sub create_rss_feed {

    my ( $days, $config ) = @_;

    my @items;
    my $todo = $config->{ days };

    for my $day ( @$days ) {

        my ( $url, $title, $description )
            = get_url_title_description( $day, $config );

        my $end_of_day = get_end_of_day( $day->{ date } );
        # RFC #822 in USA locale
        my $pub_date = $DAY_LIST[ $end_of_day->_wday() ]
            . sprintf( ', %02d ', $end_of_day->mday() )
            . $MON_LIST[ $end_of_day->_mon ]
            . $end_of_day->strftime( ' %Y %H:%M:%S %z' );

        push @items, join( '',
            '<item>',
            '<title>', escape( $title ), '</title>',
            '<link>', escape( $url ), '</link>',
            '<guid isPermaLink="true">', escape( $url ), '</guid>',
            '<pubDate>', escape( $pub_date ), '</pubDate>',
            '<description>', escape( $description ), '</description>',
            '</item>'
        );
        --$todo or last;
    }

    my $xml = join( '',
        '<?xml version="1.0" encoding="UTF-8"?>',
        '<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">',
        '<channel>',
        '<title>', escape( $config->{ name } ), '</title>',
        '<link>', escape( $config->{ 'blog-url' } ), '</link>',
        '<description>', escape( $config->{ description } ),'</description>',
        '<atom:link href="', escape( $config->{ 'rss-feed-url' } ),
        '" rel="self" type="application/rss+xml" />',
        @items,
        '</channel>',
        '</rss>',
        "\n"
    );

    my $path = $config->{ 'rss-path' };
    path( "$config->{ 'output-dir' }/$path" )
        ->append_utf8( { truncate => 1 }, $xml );
    $config->{ quiet } or print "Created '$path'\n";

    return;
}

The code is quite straightforward. The calculation of the publication date is explained in RFC #822 and RFC #3339 dates in Perl.

The escape function is also hand coded to make sure its output is identical to the escape function in Python's html module:

sub escape {

    my $str = shift;

    for ( $str ) {
        s/&/&amp;/g;
        s/</&lt;/g;
        s/>/&gt;/g;
        s/"/&quot;/g;
        s/'/&#x27;/g;
    }
    return $str;
}

It uses a for loop to alias $str to $_. As the s operator defaults to $_ this saves some typing and in my opinion is more clear.

Note that a single quote is replaced with a hexadecimal code instead of ' for maximum compatibility, see also Character entity references in HTML 4.

If you are interested in the rest of the source code you can download it from GitHub.

Hand coding an RSS 2.0 feed in Perl

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